A variety of physical impairments limit the effective transmission of data signals over wireline and wireless channels, such as the frequency selective nature of the channels, which causes different frequency components of the input signal to be attenuated and phase-shifted differently. This causes the impulse response to span several symbol intervals, resulting in time-smearing and interference between successive transmitted input symbols, commonly known as intersymbol interference (ISI). The ISI resulting from the channel distortion, if left uncompensated, causes high error rates. The solution to the ISI problem is to design a receiver that employs a means for compensating or reducing the ISI in the received signal. The compensator for the ISI is called an equalizer.
There are two general classes of equalization techniques to mitigate ISI:    (a) Maximum likelihood sequence estimation (MLSE), where a dynamic programming algorithm is used to determine the most likely transmitted sequence, given observations of the received noisy and ISI-corrupted sequence and knowledge of the channel impulse response coefficients; and    (b) Sub-optimal equalizer structures like a linear equalizer (LE), where one simple finite impulse response (FIR) filter is used to mitigate ISI, or a non-linear decision feedback equalizer (DFE) that in addition to the feed-forward FIR filter, employs a feedback filter (FBF) on the previously detected symbols.MLSE uses a sequence of received signal samples over successive symbol intervals to make decisions about the transmitted symbols, and is optimal from a bit error rate (BER) perspective. However, MLSE has a computation complexity that grow exponentially with the length of the channel time dispersion, and in most channels of practical interest, such a large computational complexity is prohibitively expensive to implement. In sub-optimal structures like LE and DFE, data detection is done on a symbol-by-symbol basis and hence is much simpler to implement than the optimal MLSE. Linear equalization uses a linear filter with adjustable coefficients. Decision feedback equalization exploits the use of previous detected symbols to suppress the ISI in the present symbol being detected.
In a typical baseband digital transmission over wireline, such as a DS3/E3/STS-1 line, the signal is distorted and attenuated due to the channel characteristics, cross talk, noise and timing jitter. Traditionally, as illustrated in FIG. 1, an analog equalizer is used at the receiver to compensate for intersymbol interference (ISI) due to the channel, and an analog timing recovery unit is used to acquire the optimal instant for sampling the received signal. This recovered timing signal is then used by digital signal processing circuitry. Referring to FIG. 1, a prior art receiver 10 uses an analog equalizer 12 and a digital timing recovery circuit comprising an analog-to-digital converter (ADC) 14, a detector 15, a timing error detector (TED) 16, a filter 17, and a voltage controlled oscillator (VCO) 18. Although the timing recovery circuit in FIG. 1 is digital overall, the VCO 18 portion is actually an analog circuit.
Techniques such as this decouple timing from equalization. The analog equalizer 12 illustrated in FIG. 1 does not require any timing information. In this way, it is able to decouple itself from timing acquisition. Unfortunately, the technique is slow to converge and will not yield as good a performance as that achieved by a straightforward linear symbol spaced equalizer (with ideal timing). Moreover, the technique depends on a power hungry analog equalizer circuit.
Other typical techniques of mitigating ISI are digital and use a symbol or partial-spaced equalization and may use a non-linear blind timing algorithm that precedes the equalizer or one that uses decision feedback. Such techniques are described in Proakis, J. G., Digital Communications, 3rd Edition, McGraw-Hill, 1995, pp. 358–365; and Razavi, Behzad, “Design of Monolithic Phase-Locked Loops and Clock Recovery Circuits—A Tutorial”, Monolithic Phase-Locked Loops and Clock Recovery Circuits, Ed. Behzad Razavi, IEEE Press, New York, 1996, 1–39. Methods that are based on the equalizer decision cannot handle input frequency offsets; as the equalizer tracks the frequency offset, the main tap slowly moves from one tap location to the next, eventually causing equalization failure. On the other hand, prior art techniques that use a blind, non-decision-based timing recovery are not very robust in the presence of high frequency input jitter.
Non-decision-based linear equalization uses a linear filter with adjustable coefficients; as those coefficients are adjusted through the equalization process, the distribution of the coefficients at any symbol interval can be used in timing recovery techniques. For example, U.S. Pat. No. 4,004,226 to Qureshi et al. discloses an automatic adaptive equalizer having taps spaced equally apart, tap coefficient circuitry for repeatedly multiplying the output of each tap by a respective tap coefficient, and adjustment circuitry for adjusting the tap coefficients to effect equalization, followed by output circuitry responsive to the equalizer for providing output signals at specific times. During each symbol interval, an algorithm identifies the largest magnitude coefficient. If the largest magnitude coefficient is determined to be the center coefficient (although it is not necessary to be precisely in the center of the coefficient queue), no timing adjustment is made. When the largest magnitude coefficient is not in the center, the receiver timing is advanced if the largest coefficient is in the back of the queue or retarded if the largest coefficient is in the front of the queue. The timing recovery circuitry comprises means for keeping the principal tap coefficient within a predetermined number of taps from the center of the equalizer.
U.S. Pat. No. 4,334,313 to Gitlin et al. and U.S. Pat. No. 4,411,000 to Kustka also utilize the location of the largest magnitude equalizer coefficient to provide timing adjustment. In U.S. Pat. No. 4,334,313, the coefficient tracking approach does not use a center coefficient; instead, the coefficient queue is divided into two portions—a front and back portion. During each symbol interval, the location of the coefficient having the largest magnitude is referred to as the “reference” coefficient. If the reference coefficient is found to be located in the front of the queue, a “retard” signal is generated. Otherwise, an “advance” signal is generated. An advance timing adjustment is performed only when the number of advance signals generated since the last timing adjustment exceeds the number of retard signals by certain amount. Similarly, a retard timing adjustment is performed only when the number of retard signals generated since the last timing adjustment exceeds the number of advance signals by certain amount. This approach is advantageous because the timing adjustment is small, therefore minimizing the possibility of an over-correction. Moreover, this method prevents any rapid timing adjustment when the reference coefficient is alternating rapidly between the front and back portions of the queue.
In U.S. Pat. No. 4,411,000, instead of having a fixed step of timing adjustment as taught by Gitlin et al., the step size of the timing adjustment varies with the location of the reference (largest) coefficient. In U.S. Pat. No. 4,411,000, the distance of the reference (i.e. largest) coefficient from a predetermined point (for example, center tap) in the coefficient queue determines the magnitude of the timing adjustment. In other words, the greater the distance, the larger the increment magnitude. The goal is to push the reference coefficient back toward the center.
U.S. Pat. No. 5,825,818 to Kaku et al. also uses a method for timing adjustment based on equalizer coefficients. After a received signal has been equalized through the adjustment of equalizer tap coefficients to eliminate distortion in the received signal, a tap-power detector is used to detect the resulting distribution of the equalizer tap coefficients, and to compute the power summations of the tap coefficients on the left and right sides of the center tap and calculate the tap-power difference between the left and right taps power summations. Then the tap coefficients on one side of the equalizer are weighted with a symbol “k” to push the reference coefficient back toward the center of the equalizer. The symbol “k” is used to compensate for an asymmetric distribution of tap coefficients, to make the distribution symmetric again.
In another example, European Patent 599311B1 discloses a time recovery technique which uses the time difference between two clock pulses, namely a receiver oscillator and a separate VCO, to select a set of pre-defined tap coefficients for the transversal filter.
Each of the above-noted patents utilize equalizer tap coefficients, after their adjustment during equalization, for purposes of timing recovery. Several patents teach methods comprising tracking shifts in the reference coefficient from the center position of the equalizer, and determining the appropriate timing adjustment based on the shifted location of the reference coefficient, with means to attempt to push the reference coefficient back to the center tap. However, such prior art timing recovery techniques do not provide as high of a resolution as might be desired. While such techniques produce a timing error whose units are in sampling intervals, or worse yet as a ±1, what is needed is a system that achieves a resolution less than the sampling interval, and preferably much less. A finer resolution is important to minimize jitter generation in the timing loop.